1. Field of the Invention
This invention relates to a fuel cell, and more particularly to an electrolyte-soluble fuel cell which employs, for example, methanol or formaldehyde as a fuel.
2. Description of the Prior Art
FIG. 1 is a schematic view in which the structure of a prior-art fuel cell employing an electrolyte-soluble fuel is shown. As apparent from FIG. 1, the prior-art fuel cell of the electrolyte-soluble fuel type has a structure in which an air electrode 2 (positive electrode) formed of a porous material such as carbon serves also as a partition wall between an electrolyte 1 with the fuel dissolved therein (hereinbelow, written "anolyte") and the air (although the air is the most suitable in practical use, any gas containing oxygen may be employed besides the air). Numeral 3 designates a negative electrode.
This structure has the disadvantage that, as the fuel cell is continued to be used, the anolyte 1 gradually leaks out through the pores of the air electrode 2.
In the prior-art fuel cell employing the electrolyte-soluble fuel, and electromotive reaction takes place at a gaseous phase interface in the electrode 2. In consequence, part of the water produced by the electromotive reaction is emitted onto the air side in the form of water vapor. The greater part of the water, however, enters the anolyte 1 and lowers the concentration of the anolyte 1. Therefore, the ratios of the components (water, electrolyte and fuel) of the anolyte vary with the use in a complicated manner. In order to keep them constant, the water must be removed while measuring the composition ratios of the anolyte. This requires an extremely intricate operation.
Moreover, since the air and the anolyte come into contact, oxygen in the air oxidizes the fuel in the anolyte by the air electrode catalysis in a non-electrochemical manner. As a result, the utilization factor of the fuel is reduced.
There has been proposed a fuel cell in which, to the end of preventing the liquid leakage from the air electrode, an oxygen transmitting film made of, for example, polyethylene is arranged outside the air electrode (on the air side) or in the air electrode (Japanese Patent Application Publication Nos. 26896/1973 and 12773/1971).
However, the oxygen-transmitting films of polyethylene, etc. presently known exhibit low transmission factors for oxygen and cannot supply a sufficient quantity of oxygen to the surface of the air electrode. Accordingly, the fuel cell employing such a film is difficult to provide a satisfactorily high output. On account of the direct contact between the oxygen and the anolyte, the concentration of the anolyte is lowered by the produced water as described above, and besides, the anolyte is oxidized, the composition of the anolyte varying also due to this effect.